(a) Field of the Invention
The present invention relates to a supported metallocene catalyst used for preparing polyolefin whose physical properties and molecular weight distribution can be easily controlled, a method for preparing the same, and a method for preparing polyolefin using the same, more particularly to a supported metallocene catalyst wherein at least two kinds of metallocene compounds are supported on a metal oxide such as silica, a method for preparing the same, and a method for preparing polyolefin using the same.
(b) Description of the Related Art
The metallocene catalyst system comprises a main catalyst whose main component is a transition metal compound, mainly a Group IV metal, and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst offers a polymer having a narrow molecular weight distribution depending on the single site characteristics. The molecular weight and molecular weight distribution of polyolefin are important factors that determine the fluidity and mechanical properties that affect the physical properties and processability of a polymer. To manufacture a variety of polyolefin products, it is important to improve melt processability through molecular weight distribution control (C. A. Sperat, W. A. Franta, H. W. Starkweather Jr., J. Am. Chem. Soc., 75, 1953, 6127). Especially for polyethylene, toughness, strength, environmental stress cracking resistance (ESCR), and so forth are very important. Therefore, a method of preparing a polyolefin having a bimodal or broad molecular weight distribution to enhance mechanical properties and processability has been proposed.
The polyolefin having a bimodal and broad molecular weight distribution is a polymer that has two average molecular weights, one of which is higher and the other lower. Conventionally, there have been many attempts to prepare a polymer having such a molecular weight distribution. They can be categorized into the following three groups.
First, a method of preparing two polyolefins having different molecular weights by post reaction or mixing has been proposed (U.S. Pat. No. 4,461,873). However, such physical mixing requires additional production cost. Also, because the gel content is high due the compatibility of the two polymers, it cannot be applied to manufacturing containers and films.
Second, a method of preparing a polyethylene having a broad molecular weight distribution or a bimodal molecular weight distribution by differing injected amounts of hydrogen gas and comonomers using more than two multistep reactors has been proposed (C. A. Sperat, W. A. Franta, H. W. Starkweather Jr., J. Am. Chem. Soc., 75, 1953 6127, N. Kuroda, Y. Miyazaki, K. Matsumura, K. Jubo, M. Miyashi and T. Horie, 1979 Ger. 28,856,548). Although this method can solve the gel problem, the process is complex and the operation cost is high.
The third method, a method of mixing two different catalysts in a single reactor or applying more than two catalysts to a single support, is more useful than the above two methods. Most patents and literatures mention a metallocene compound and a Ziegler-Natta titanium compound supported on a single support (U.S. Pat. No. 6,444,605 B1, U.S. Pat. No. 6,399,531 B1, U.S. Pat. No. 6,417,129 B2, U.S. Pat. No. 6,399,723 B1, U.S. Pat. No. 5,614,456, Korea Patent Publication No. 1988-045993, Korea Patent Publication No. 1998-045994, Korea Patent Publication No. 1999-022334, Korea Patent Publication No. 2000-0042620).
A polyolefin prepared by this method has limited physical properties due to the specific polymerization characteristics of the Ziegler-Natta catalyst and the metallocene catalyst. For instance, when preparing a polyolefin using a metallocene catalyst, the stereoregularity, copolymerization characteristics, molecular weight, crystallinity, and so forth of the obtained polymer can be controlled by changing the ligand structure of the catalyst and the polymerization condition.
Accordingly, a polyolefin having ideal molecular weight distribution, stereo regularity, copolymerization characteristics, and so forth can be prepared by using several metallocene catalysts having specific characteristics. For the preparation of a polyolefin having a bimodal or broad molecular weight distribution using a metallocene catalyst, a method of using two different metallocene catalyst systems having different growth rate and termination rate constant has been proposed (J. A. Ewen, Stud. Surf. Sci. & Catal. vol. 1986 25, U.S. Pat. No. 6,384,161 B1). The Kaminsky group of Germany has reported that the molecular weight distribution (Mw/Mn) of ethylene polymerization has broadened from about 1.9 to 2.3 to about 4.1 to 10.0 when they used a hybrid metallocene catalyst system such as Cp2ZrCl2—Cp2HfCl2—MAO (A. Ahlers, W. Kaminsky, Makromol. Chem., Rapid Commun., 9, 1988 457).
However, above-mentioned literatures and patents only disclose the polymerization using two catalysts. They just mention selection of organometallic compounds and homogeneous catalyst systems for controlling the molecular weight distribution and polymer properties, and they do not offer supported catalysts applicable to gas-phase and slurry processes.
The industrial polyolefin production processes can be classified into the high-pressure process, the solution process, the slurry process, the gas phase process, and so forth. With the development of the metallocene catalyst, there have been many attempts to produce polyolefin using these processes and just replacing the catalyst. Until recently, the metallocene catalyst is mostly used for the solution process, the gas phase process and the slurry process. The gas phase and the slurry processes require apparent density control of the polymer to improve the production unit per unit volume of the reactor. In addition, the fouling problem, or sticking of the polymer on the reactor wall, should be solved. The most common method for increasing the apparent density and solving the fouling problem is to fix the homogeneous metallocene catalyst on a solid support such as silica, alumina or a metal oxide.
Among them, silica is the most frequently used as a support. When a silica support is dried at high temperature, the hydroxyl group on the surface is removed as water and a siloxane group is obtained, as in the following Scheme 1. If the drying temperature is 200 to 500° C., only the easily removable hydroxyl group (—OH) is reversibly removed as water and the less reactive siloxane group is obtained. However, if the drying temperature is above 600° C., even the hardly removable hydroxyl group is forcibly removed as water and the siloxane group with a large ring strain and high reactivity is obtained (I-S. Chuang and G. E. Maciel, Journal of American Chemical Society 118, 1996, 401). The highly reactive siloxane group obtained by drying at the temperature over 600° C. reacts with an alkoxy silane group, as in Scheme 1 (J. Blmel, Journal of American Chemical Society 117, 1995, 2112; L. H. Dubois, Journal of American Chemical Society 115, 1993, 1190).

Here, the figure on the left is the ordinary silica, the figure on the center is the silica having a highly reactive siloxane group, which has been dried at a temperature over 600° C., and the figure on the right is the silica attached with an alkoxy silane group.